The present invention relates to a method for automatically identifying heavy uphill drive conditions and select appropriate gear ratios in a heavy vehicle with an Automatic Mechanical Transmission.
The invention also relates to a computer program for carrying out said method.
Automatic transmissions of the Automatic Mechanical Transmissions (AMT) type have become increasingly common in heavy-duty vehicles as microcomputer technology has continued to advance and has made it possible, with a control computer and a number of control elements, for example servo motors, to precision-control the engine speed, the connection and disconnection of an automated clutch between engine and gearbox and coupling members of the gearbox, relative to one another, so that smooth gearshift is always obtained at the correct rev speed. The advantage with this type of automatic transmission compared to a traditional automatic transmission based on a set of planetary gears and with a hydrodynamic torque converter on the input side is firstly that, particularly as regards use in heavy vehicles, it is simpler and more robust and can be produced at substantially lower cost, and secondly that it has higher efficiency, which means the prospect of lower fuel consumption.
The automatic mechanical transmissions of the type mentioned above strongly reduces the drivers gear shifting work and the driver usually will not need to worry about selecting the right next gear ratio when driving the vehicle. All this is taken care of the AMT. Even though AMT:s usually are programmed with gear shifting strategies that gives a satisfying gear selection for a great deal of the total vehicle drive situations, there still can occur situations where the selection of gears is not optimal and, in some particular cases this can lead to a difficult situation for the driver and the vehicle might even come to a stop and be unable to restart. This may be due to the limited friction at the driving wheels.
An example of such situation is when a heavy loaded vehicle is driven in a heavy uphill drive condition. Usually the continuous adaptation of the automatic selection of gears when driving is based on predetermined gear shift maps, where accelerator pedal position and engine speed are used. The gear shift maps commonly tuned to achieve a suitable engine speed after completed gear shift. The AMT-system simply tries to select a gear that is best for a particular situation. In this context it should be mentioned that some AMT-systems according to state of the art selects the best gear only based on current circumstances. There are also AMT-systems with different kinds of predictive functions for enhanced gear selection, which means that several future gear shift steps, i.e. a gear shifting sequence, can be predicted.
This means that when a heavy vehicle starts to climb an uphill road, sensing increased travel resistance, the programmed gear shifting strategy will typically make the AMT to shift down a certain number of gear steps depending on current traction power and acceleration (i.e. in an uphill state usually retardation).
As the vehicle travels further and the inclination of the uphill increases and the travel resistance increases even more, the shifting strategy will make the AMT shift down some more gear steps and one of the highest gear ratios of the AMT will be engaged. This selected high gear ratio is based on the predetermined gear shift maps.
For a given road incline there is a lowest gear ratio, where the vehicle is able to climb the road incline. If the AMT-system after a down shift ends up in a higher gear ratio than said lowest gear ratio, the vehicle speed will be lower than necessary, which means increased fuel consumption and decreased average speed. The low vehicle speed can even cause traffic jam. Another drawback of this situation is that there is a high risk that the driven wheels of the vehicle will slip, due to the heavy vehicle, high gear ratio and thus high output torque. Slip can occur even when the vehicle is driven on a dry asphalt surface. If one or more of the driven wheels starts to slip the risk for a full stop in the middle of the steep uphill is high. If the vehicle stops due to slip, the driver will have a problem to take off. The driven wheels will slip, when trying to take off, which means that the driver will have to reverse the vehicle down the steep uphill.
A typical example of a vehicle can be a truck with a trailer with a gross train weight of e.g. 60 tons. The truck can e.g. be equipped with an AMT with 14 forward gears (the two lowest gears can be crawl gears) of the type disclosed e.g. in WO02/064996.
FIG. 1 shows diagrammatically a typical steep uphill road with e.g. 15% incline and a length of approximately 150 meters from horizontal position A to elevated horizontal position B. The driver will slightly increase the depressing of an accelerator pedal arranged in the 60 tons truck when the driver sees the approaching steep uphill, thus demanding more torque.
A typical state of the art gear shift strategy for said 60-tons truck would do gear shifting as follows when passing position A with a vehicle speed of 35 km/h and 6th gear engaged. At point C the vehicle senses the inclination of the road and the retardation. This initiates a downshift. Since the vehicle is heavy, the road is steep and the retardation relatively high, the programmed shifting strategy will make the AMT shift down more than one gear step. In this case it will typically down shift from gear 6 to gear 4. The two-step down shift guarantees that the AMT will not “miss” a selected gear due to too high retardation during driving torque interruption caused by disengagement of vehicle engine and driven wheels of the vehicle (caused by e.g. clutch disengagement or putting AMT in neutral). The selected 4th gear is engaged successfully, but the retardation of the vehicle continues.
A gear will be missed if a selected gear, that the AMT tries to engage, can not be engaged. It might be a too high gear (i.e. too low gear ratio) for the current vehicle speed. The high vehicle retardation during the gear shift may be the reason for not being able to engage the gear. A missed gear could under these circumstances lead to a stop of the vehicle.
In position C a downshift is initiated as mentioned above and at position D the AMT has engaged gear 4 and a higher output torque is transferred to the driven wheels of the vehicle and the retardation is not as high as in position C, but the retardation continues. At position E the engine speed and the vehicle speed is so low that the AMT initiates another downshift. Again a multiple step down shift is selected to secure that a selected gear will not be missed. In this case gear 1 is selected, which means that there will at least theoretically be more than enough torque transferred to the driven wheels so that the truck can pull through the uphill all the way up to position B. To sum up the example as above the selected gear shifting sequence during the heavy uphill drive condition was 64-1.
If the friction between the road surface and the tires of the driven wheels is enough the truck will pull through all the way to position B with gear 1 engaged. If the driven wheels starts to slip, the vehicle will loose propulsive power and the vehicle will eventually stop in the middle of the uphill and will have severe problems to take off.
It is desirable to select a gear in a heavy uphill drive condition that is more optimized for the situation, so that average speed can be increased and fuel consumption lowered, and if a situation occurs where the driving wheels of the vehicles will slip, select a gear that minimizes the slip risk and at the same time that the vehicle can manage to pull through the whole uphill.
The method according to an aspect of the invention is a method for gear selection during driving of a vehicle in a heavy uphill drive condition, said vehicle comprising an engine with an engine output shaft connected to an automated mechanical transmission, a transmission output shaft connected to at least one driven wheel of the vehicle, at least one control unit for receiving input signals including signals indicative of vehicle speed, engaged ratio of said transmission, rotational speed of said engine, rotational speed of said input shaft and displacement of a throttle control for engine torque request, and for processing said signals in accordance with programmed logic rules to issue command output signals to said engine for torque request, to said transmission for gear shifting and to said clutch for engagement/disengagement. The method is characterized in that when sensing a heavy uphill drive condition a target gear is determined for said heavy uphill drive condition, said target gear being the highest possible gear with lowest possible gear ratio where the vehicle, in view of at least current circumstances, will be at least theoretically able to hold a constant vehicle speed or accelerate at least slightly, and where further selection of downshifts will be adapted so that no lower gear than said target gear will be selected and engaged.
The advantage of the method according to the invention is that the determined target gear is an optimized gear that gives, for a particular sensed heavy uphill drive condition the lowest fuel consumption, highest average speed, minimized risk for drive wheel slip and that the vehicle still at least theoretically will be able to pull through said heavy uphill drive condition, i.e. hold a constant vehicle speed or accelerate at least slightly the whole uphill (according to at least current sensed condition).
When a target gear has been determined, the further downshift or downshifts will be adapted to said target gear so that a lower gear than the target gear will not be engaged for a certain heavy uphill drive condition.
According to one embodiment of the method according to the invention said registration of said heavy uphill drive condition is a result of sensing at least current road inclination, current vehicle speed, current throttle control position and vehicle gross weight. Most AMT-systems already use said parameters for gear selection and there will in most cases only be needed new software to implement the functionality for identifying heavy uphill drive condition according to the invention.
According to another embodiment of the method according to the invention said determination of the target gear includes at least parameters: available vehicle engine power, vehicle gross weight, current road inclination, current vehicle speed and available total gear ratios.
According to a further embodiment of the method according to the invention, when no desire to climb the uphill anymore condition is sensed, the downshift limitation to said target gear is cancelled. The vehicle will be able to stop or drive slower than admissible with the target gear engaged.
In a further development of the last embodiment above of the invention, the “no desire to climb the uphill anymore condition” is a result of sensing at least a partly or a full release of said throttle control. The system must be able to cancel the downshift limitation to said target gear if e.g. a driver of the vehicle for some reason wants to stop or at least decrease the vehicle speed. In a further development of this embodiment said at least partly release of the throttle control is at least a 15% displacement of total displacement of the throttle control.
According to a further embodiment of the method according to the invention, the control unit senses a change of the heavy uphill drive condition during said heavy uphill drive condition and the selection of said target gear will be either updated or cancelled accordingly.
Further advantageous embodiments of the invention are described herein.